The Nimrod was designed as a sea patrol and anti-submarine aircraft and entered service with the Royal Air Force in the UK over 30 years ago.

Its design is based on the de Havilland Comet, the first commercial jet airliner, with four jet engines integrated into the wings next to the fuselage.

The maritime versions are often used for long-range search and rescue.

But the R1, which entered service with the RAF in 1974, has a surveillance role with its original equipment removed and replaced with systems for electronic eavesdropping.

The plane has a top speed of 575 mph (925 kph) and a range of 5,755 miles (9,262 km) without refuelling. It can therefore loiter over a target area for long periods at up to 42,000 feet (12,800 metres).

During flight, two of the engines can be shut down for long-range cruising and there is normally a crew of 12 on board - two flight crew and 10 specialist equipment operators.

There are only three R1s in operation, operated by 51 Squadron, based at RAF Waddington in Lincolnshire.